Conventionally, a parking brake is used for restricting movement of a parked vehicle. For example, a manual parking brake is used in which an operation force applied to an operation lever is transmitted to a brake mechanism of the parking brake when the operation lever pulls a brake cable caused by the operation force. An electrically driven parking brake is also used in which a rotational force of a motor is transmitted to the brake mechanism of the parking brake by pulling a cable making use of the rotational force of a motor, and the like.
In an EPB which is the electrically driven parking brake, when the brake is locked, a motor is rotated to a lock side (forward rotation) and a motor rotational force is transmitted to a brake mechanism (actuator) as well as the motor is stopped in a state that a brake force is generated, whereas when the brake is released, the brake force is released by rotating the motor to a release side (reverse rotation).
In the lock/release control, a desired lock state is maintained in such a manner that when a motor current reaches a target current value (motor cut current) at the time the brake is locked, the motor is stopped to the lock side.
However, the relation between a motor current and an actuation force generated by a brake mechanism for converting a force resulting from rotation of the motor to a brake force is varied by a condition such as a temperature and the like. The actuation force corresponds to the brake force and includes a force for pulling a cable, a force for depressing a brake shoe or a brake pad by the pulled cable in addition to the brake force itself. To cope with the above problem, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-515344 proposes a method of separating an idle current, which flows when a motor rotates without load, and a current consumed to generate an actuation force and determining a target current value only by the current consumed to generate the actuation force. Specifically, a motor temperature and an idle speed which is a rotation speed when a motor rotates without load are detected by sensors and the target current value is corrected according to their values.
However, in a method shown in the Japanese Unexamined Patent Application Publication No. 2007-515344, the relation between a motor temperature and a motor idling speed and correction values is previously determined as a function or a characteristics curve, and the correction values corresponding to the motor temperature and the motor idling speed detected by a sensor are set based on the function or the characteristics curve determined previously. Accordingly, all the correction values are set using the function or the characteristics curve which are previously determined as a premise. However, since the function and the characteristics curve are dispersed according to the dispersion of individual units of the motor, a problem arises in that the method shown in Japanese Unexamined Patent Application Publication No. 2007-515344 cannot make a correction in response to the dispersion of the individual units of the motor. Further, there is also a problem in that the sensor for detecting the motor temperature and the motor idling speed must be additionally provided.